Diode-lasers are efficient devices for converting electrical power into coherent optical power. An edge-emitting diode-laser has a diode-laser emitter, which is a type of waveguide laser-resonator, grown on a single-crystal substrate. The diode-laser emitter emits laser-radiation through an end facet in an emission direction. For high-power applications, a diode-laser bar having a plurality of diode-laser emitters provides a convenient way to scale the optical power available from a single diode-laser emitter. A diode-laser bar has typically between 10 and 60 such diode-laser emitters spaced apart and arranged in a “horizontal” linear array thereof. For further power scaling, a plurality of diode-laser bars can be stacked “vertically” to make a two-dimensional array of diode-laser emitters. Diode-laser bars arranged in this manner are typically referred to as a “vertical stack”.
A diode-laser bar includes a plurality of semiconductor layers epitaxially grown on the substrate, with the diode-laser emitters defined in the epitaxial layers. Typically, the substrate is an n-type substrate, and layers are grown such that layers forming the “p-side” (anode-side) of the diodes are uppermost.
The term “packaging” applied to diode-laser bars refers to mounting a diode-laser bar, or an array of diode-laser bars, on some sort of cooling-base or heat-sink. The base is usually made of copper. In a “conductively cooled package” (CCP) the base has sufficient mass to remove waste heat from the diode-laser bar. For higher power operation, the base is typically water-cooled, for example through a micro-channel arrangement. The diode-laser bar is soldered “p-side down” either directly onto the base or via a submount. The submount is made of a material having a coefficient of thermal expansion (CTE) between that of the substrate material and the base material, generally a material having a CTE close to that of the substrate material.
Electrical connection to the diode-laser bars places the base, and cooling-water therein, in electrical contract with the diode-laser bar energizing circuit. In an array of diode-laser bars, the water can short-circuit electric connection to the bars, unless the electrical conductivity of the water is low. A common solution to this is to use de-ionized (DI) or high-resistance water. However, DI water is more corrosive on metals than water from conventional building supplies. The use of DI water is also expensive and inconvenient.
Even small “stray” currents through the cooling water, between metal elements at different electric potentials, can cause metal corrosion through galvanic action. In addition to erosion of metal elements, particles produced by galvanic action can block cooling-channels in micro-channel cooled arrangements, which have typical internal dimensions of about 0.5 millimeters (mm) or less. Plating the cooling-water channels with a metal such as gold can mitigate such corrosion. However, plating internal channels by immersion-plating (usually using forced-flow plating solutions) results in uneven plating that is difficult to inspect for quality assurance.
There is a need for an improved diode-laser bar assembly, having the cooling-water electrically isolated from both the n-side and p-side electrical potentials. Such an assembly should preferably not require the use of de-ionized water.